Amplifier system



g- 20, 1940- J. HARDWICK ET AL ,203

AMPLIFIER SYSTEM Filed June 30; 1936 I r0 r0 SOl/PCE- 40 40 0/-' SIGNALS (/RCU/T Z 70 7 r0 sou/e65 0A0 0F SIGNALS CIRCUIT INVENTORS JOHN HARDWICK ERIC, c. WHITE I ATTORNEY Patented Aug. 20, 1940 UNITED I v'sm'rss AMPLIFIER SYSTEM John Hardwick, West Drayton, and Eric Lawrence Gasling White, Hillingdon, England, assignors to Electric & Musical Industries Limited,- Hayes, Middlesex, England Application June 30, 1936, Serial No. 8,094 In Great Britain'July 9, 1935 4 Claims.

The present invention relates to electric circuits for modifying frequency response in amplifiers and the like. r l

For certain purposes, for example in the transmission of television picture signals, it is required to provide an amplifier or other circuit arrangement whichfavours high frequencies by transmitting the lower frequencies with relatively less amplification or more attenuation. Insuch cases it is also required to avoid substantial phase shift or at least to avoid phase shift which is other than a mere time displacement of the signals as a whole.

It is an object of the present invention to provide a circuit arrangement such that both of the requirements set forth above are fulfilled.

The present'invention accordingly provides a circuit arrangement such for example as an amplifier in which means arranged to favour certain frequencies of applied signals relatively to other frequencies are provided for effecting frequencyresponse correction wherein, in order to avoid substantial phase shift, there are provided two or more stages each arranged to effect a part of the desired frequency-response correction, the

arrangement being such that distortion of the wave form of said signals due to phase shift introduced by one of said stages is neutralized or substantially reduced by a compensating phase stages. r

The invention will be described as applied to the correction of the fall in response at high frequencies which manifests itself in certain forms of television apparatus due to the finite size of a scanning aperture; it is to be understood that when scanning is by sweeping a spot of light over an object, or a cathoderay spot over a mosaic screen, the dimensions of the scanning spot are the equivalents of those of a scanning aperture.

In the course of the following description, reference'will be made to the accompanying drawing, which illustrates. in Figswl and 2, two arrangements according to the invention. The response R of a photo-cell, in conjunction with a scanning aperture of diameter d, to a sinewave variation of object brightness of wave length x, is proportional to 55 causes an electrical oscillation of frequency w to be set up in the output circuit of the photo cell.

shift introduced by one or more of the remaining The response R is, therefore, equal to zero at the frequency -i c l 0"- d Furthermore, in the neighborhood of the frequency fo, the response will be below that at lower frequencies.

. Now

Ksing can be expressed as aseries 1.1L K .1 and hence .R is approximately proportional to A 1 1- -K2 where A is a constant. It follows that in order to correct for the fall in response as the frequency In is approached, it is necessary to introduce a correction which varies approximately with the square of the frequency.

Reference is now directed to Fig. 1 of the accompanying drawing, which shows an amplifier in which means are provided for introducing a correction of the desired character; the amplifier comprises two amplifying valves 3 and 4 which are both of the screen grid type. The picture signals to be corrected are applied to the control grid of the first valve 3 through a condenser 5. The anode of the valve 3 is connected to the positive terminal .of a source (not shown) of anode current, the negative terminal of which is earthed, through a coil I having an inductance L in series with a resistance 8 of value R1. The anode of the first valve 3 is also connected to the control grid of the second valve 4 through a coupling condenser 9, and the control grid of the second valve 4 is connected to the cathodes of the two valves and to earth through a grid leak l0 anda biasing source H. The anode of the second valve 4 is connected to the positive terminal of the anode current source through the primary winding of a transformer I2 and a resistance i3 of value R2 arranged in series, the transformer Winding being arranged between the resistance l3 and the anode ofvalve 4. One end of the secondary winding of the transformer I2 is connected to the end of the primary winding remote from the anode of valve 4, and the other end of the'secondary winding is connected to an output terminal M. The windings of the transformer have a mutual inductance M, and it is arranged i that Now in a television system, the frequency in may be 4 megacycles or more and it is desired the magnitude of L and R1 are arranged to have such values that the circuit 1, 8 and the anode-toearth capacity is always working well below its resonant frequency.

The output from the first valve of the circuit above described is proportional to (Ri-l-jwL), and it will be noted that the output is dependent, both in amplitude and phase upon the frequency. The output from the second valve is, however, proportional to the amplitude of output being proportional to a constant plus the square of the frequency, as required, but the resultant phase angle thereof being zero.

The correction thus obtained in the two-stage amplifier described is accordingly of the required form and shows no phase change.

In an alternative arrangement according to the present invention, which will be described with reference to Fig. 2, the correcting networks are associated with a single stage of amplification, but the correction is nevertheless carried out in two stages, one in the grid-cathode circuit of the valve and the other in the anode circuit.

Referring to Fig. 2, the valve 15 is again a tetrode and the signals to be corrected are applied between the control grid and earth by means of condenser I6, and grid leak l1 and biasing battery l8. The cathode of 'valve [5 is connected to earth through a resistance l9 of value R1 havinga condenser 20 of value C1 connected in parallel. The anode circuit of valve 15 includes a network l2, 13 connected and arranged as described with reference to Fig. 1, the value of resistance 13 being R2 and the mutual inductance between the transformer windings being M. The

negative terminal of the anode current source (not shown) is connected to earth as in Fig. 1, and it is arranged that It can be shown that, in the arrangement of Fig. 2, if an alternating potential difference of constant amplitude is applied between the control grid of valve 15 and earth, the current in the valve increases with increase in frequency of the applied potential difference'in accordance with the law (1-7wC'1R1) provided that i 9 issmall compared to R1 and where g is the mutual conductance of the valve.

Since the impedance of the anode circuit inthe total resultant phase shift is zero.

The increase of amplification with increase of frequency is limited in the cathode circuit as the impedance of condenser C approaches This is usually desirable, provided that limitation occurs outside the operating frequency range. The impedance of circuit 1, 8 in Fig. 1 may be limited if desired by shunting inductance 8, or the whole circuit 1, 8, by a resistance of suitable value. Similarly, in the arrangement of either of Figs. 1 or 2, a limiting resistance may be arranged in shunt either with the primary winding of the transformer, or with this winding and resistance l3 in series; such a limiting resistance is shown at 2|. in Fig.2.

The invention is not limited to television amplifiers, and many further applications of the invention will occur to those versed in the art.

Having now described our invention, what we claim and desire to secure by Letters Patent is:

1. An amplifying system comprising means to feed signals accompanied by frequency and phase distortion to a thermionic amplifier, and substantially a non-capacitive impedance means associated with the thermionic amplifier for further increasing the frequency distortion and neutralizing the phase distortion of the signals.

2. In combination, a thermionic tube having at least a cathode, a control electrode, and an anode, a parallelly connected resistor and condenser connected between a cathode and an output terminal, means to supply signalling energy having components of dilferent frequency to the control electrode and said output terminal, means to supply energy serially through a plurality of non-capacitive impedances to the anode, means for deriving energy both inductively and direct ly from a junction point of said plurality of impedances whereby the derived energy has a phase shift which is linear with respect to frequency and terminal means connected to said last named means. 1

3. .A two-stage amplifier comprising a first stage having a non-linear frequency response and a non-linear phase shift response, a second stage having a non-uniform frequency response and a non-linear phase response, the phase response of said second stage having a value complementary to said first phase response, whereby the overall phase response-is linear.

4. An amplifier system comprising a thermionic tube having at least a cathode, a control electrode and an anode, a non-linear frequency andphase shift response means including a capacitive reactance circuit connected between said cathode' and said control electrode, and a phase distortion neutralizing substantially non-capacitive imped-i ance circuit having. a non-linear frequency response connected to said anode.

JOHN HARDWICK. 

